This invention relates to drive circuits for motors having a plurality of phases, especially but non-exclusively for those for use in Electric Power Assisted Steering (EPAS) systems.
Electric Power Assisted Steering systems are well known in the prior art. The power generated by an electric motor is used to generate an assistance torque in the steering column in the same direction as a detected demand torque generated by a driver of the vehicle turning a steering wheel. For example, a gearbox may provide a connection between the motor and a steering column shaft, or directly onto a portion of a rack and pinion mechanism forming part of the steering mechanism.
The electric motor is used to assist a driver in applying torque to the steering mechanism, by applying an assistance torque of the same sense, to make it easier to turn the steering wheel, for example during parking maneuvres. Thus, operation of the motor may assist in rotating the steering column shaft, or moving a portion of the steering rack mechanism. Of course, the motor may be connected to any part of any typical steering mechanism as long as it can provide an assistance torque to aid the driver in turning the steering wheel.
A typical permanent magnet electric motor used in an EPAS system comprises a rotor that is magnetic, for example including a permanent magnet, and a stator including a plurality of phase windings on a yoke. Applying suitable voltages across each of the phase windings causes current to flow through the windings, generating a current flux vector in the air gap between the stator and the rotor. This current flux vector interacts with the magnetic field of the rotor to cause the rotor to rotate to a point of equilibrium in which the current vector is aligned with the axis of the rotor magnetic field.
To cause the rotor to turn continuously, the current passed through the windings must be varied in a sequence. This causes the current flux vector to rotate. This can be achieved by modulating the voltages across each winding under the control of a motor drive circuit.
The motor may be controlled by motor control means comprising a drive circuit, which is operative to supply a current from a power supply to the motor phase windings. In one arrangement, each phase is connected to a positive terminal of the power supply by a top transistor, and to a negative terminal by a bottom transistor, the two transistors defining an arm of a multiple arm bridge. This bridge forms the drive circuit, while the control circuits are provided by a microprocessor or similar device.
The microprocessor is operative in response to signals from a torque sensor provided on the steering column to measure the torque applied by the driver, from a motor rotor position sensor providing information about motor speed and direction and optionally from signals corresponding to current flowing in the motor bridge or power supply. This information can be used in combination with the torque sensor signal and/or column position sensor signal to determine which phase winding should be energised and when. The microprocessor produces control signals which energise the transistors of the drive circuits to cause current to flow in a desired motor phase. Such a drive circuit can be seen in European Patent Application Publication number 0 857 135.
The torque developed in the motor is dependent firstly on the current passing through the windings, in a generally linear manner, and secondly on the phase of the current relative to the flux due to the rotor magnets.
The torque required from EPAS systems is increasing as vehicles become heavier and such systems are fitted to larger vehicles. The maximum torque generated by a multi-phase brushless motor is dependent on, amongst other factors, the supply voltage. It is known to boost the voltage from the battery using a voltage boost circuit. Such circuits enable the voltage to be temporarily boosted so that the effective voltage as applied to the motor phase terminals is higher than the basic battery voltage. However, as a higher voltage is then chopped by the bridges of the drive circuit, more shielding is required against electromagnetic (EM) radiation of the drive signals.
It is also known to advance the phase of the drive signals with respect to the rotor position in order to increase the torque generated by such a motor. However, this reduces the efficiency of the motor.